SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

4.2.P-NJUNCTION IN EQUILIBRIUM 149

Neutral p-region Neutral n-region

negati v el y

charged

region

posi ti v el y

charged

region

Wp^0 Wn

(a)

E E l ectri c f i el d di recti on

Diffusion particle flow D i f f usi on current f l ow

Drift particle flow Drift current flow

Diffusion particle flow D i f f usi on current f l ow

Drift particle flow Drift current flow

Holes

Electrons

{

{

(b)

+++

+++

• ––+++


• ––


• ––

Figure 4.2: (a) Region of ap-njunction without bias, showing the neutral and depletion ar-
eas. (b) A schematic showing various current and particle flow components in thep-ndiode at
equilibrium.

The ratio ofμpandDpis given by the Einstein relation

μp

Dp

=

e

kBT

(4.2.2)

Since the Fermi level is uniform in the structure as we go from thep-side to then-side, as shown
in figure 4.3. As a result of bringing thepandntype semiconductors, abuilt-involtage,Vbi,is
produced between then−side and thep−side of the structure. As indicated in figure 4.3, the
built-in voltage is given by

eVbi=Eg−(Ec−EF)n−(EF−Ev)p